Infrared imaging technology utilizing cooled arrays of either intrinsic or extrinsic semiconductor photodetectors is already in an advanced state of development. They have a sensitivity sufficient to permit real-time imaging of high quality to be achieved from objects at normal ambient temperatures. Infrared imagers also have applications in space where both man-made and celestial objects are being viewed against a very dark background.
With respect to two-dimensional arrays of imaging devices for use with visible light, a number of different types of devices have been proposed for different applications requiring different combinations of size, image quality, effective speed and cost. In particular, CCD (charge-coupled device) technology has been useful in the design and construction of monolithic arrays of visible imaging detectors. Visible light photons falling on the individual pixels (picture elements) during an appropriate "integration period" results in an increase in the stored charge associated with each pixel which may then be "read out" by the CCD.
Semiconductor device fabrication techniques for use with a silicon substrate have been under development and in commercial use for many years. It is the existence of this wealth of device technology (whereby metal-oxide-semiconductor (MOS) elements may be fabricated on a silicon subtrate so as to have practically any desired electrical characteristic) which has made silicon fundamental to a large class of imaging devices.
However, the individual photons in the infrared region have substantially less energy than those associated with the visible and ultraviolet (UV). Accordingly, the band gap associated with intrinsic silicon is too large to permit intrinsic silicon detectors to be used in the infrared. On the other hand, it is not presently feasible to adapt existing silicon-based visible (or UV) detectors and detector arrays to the infrared region merely by constructing them on a substrate having the required small energy gap because, at present, there is a lack of suitable device fabrication technology for such candidate materials.
It has been proposed to construct photocapacitive extrinsic silicon infrared detectors on a monolithic silicon substrate by doping certain active regions of the silicon substrate with shallow impurities having an energy level very close to one of the band edges. Phosphorous and boron are among the dopants that have been proposed for such devices, photodetection resulting in the generation of free carriers of one type only. Such devices are not readily adaptable for use with conventional X-Y addressing schemes because there is no way provided to replace the excited free carriers lost by the dopant so as to maintain electrical charge neutrality at the dopant sites.
It is accordingly an important object of the present invention to provide a photodetector device that can be fabricated as a two-dimensional array of X-Y addressable picture elements for use in an infrared camera or other suitable imaging application.
It is another object to provide an infrared photodetector utilizing silicon as a substrate whereby existing metal-oxide-semiconductor and charge-coupled device technology may be employed to fabricate a monolithic array.
It is another important object to provide a source of replacement carriers to replace the carriers excited from the dopant region of the extrinsic infrared detectors so as to maintain charge neutrality at the dopant sites.
Yet another important object of the present invention is to provide an infrared detector device wherein an external connection provides a path for replacing the carriers which have been excited by the incoming radiation.
Yet another important object is to provide an infrared detector device wherein an electrode generates a fringing field which causes the dopant band in an adjacent detector area to be "tipped" away from the equilibrium Fermi level so as to enhance the flow of excited carriers into a drain channel.
Yet another object is to provide an X-Y addressed array of infrared detector devices requiring only one gate electrode per line of such devices.
Still another object is to provide an infrared detector device which is defined by a channel stop which surrounds the detector so that no electrode is required on top of the detection region and that accordingly the device may be illuminated either from the top surface or from the rear surface.
Many types of prior art imaging devices are discussed at length at pages 241 through 295 of the book Charge-coupled Devices and Systems edited by M. J. Howes and D. V. Morgan and published in 1979 by John Wiley & Sons Ltd., Chichester, England. That discussion, and in particular section 5.4 thereof entitled "Infrared Imagers", is incorporated herein by reference, both as evidence of state of the prior art and for any assistance it may give the average artisan in making and using the present invention.